Prior art can be separated into four groups according to the type of lenses used to collect solar radiation, as follows;
1. Spherical Lenses. This group includes Great Britain Pat. No. 18,998 of Keun, Great Britain Pat. No. 375,685 of Ylla-Conte, and U.S. Pat. No. 3,934,573 of Dandini. Clegg's hemispheric solar panel will belong to this group.
2. Elongate, Rectilinear Convex Lenses. This group includes U.S. Pat. No. 4,267,826 of Hitt, Great Britain Pat. No. 2,100,415 of Subryan and Japan Pat. No. 21749 of Akane.
3. Conical Lenses. This group includes U.S. Pat. No. 4,325,612 of Clegg and U.S. Pat. No. 4,333,713 of Clegg.
4. Miscellaneous Lenses. This group includes U.S. Pat. No. 2,259,902 (toroidal lens) of McCain, U.S. Pat. No. 4,411,490 (fresnel lenses) of Daniel, and French Pat. No. 2,446,450 (prismatic lenses) of Bel Hamri.
Considering the spherical lens art of the first group, Keun uses a large biconvex lens to project concentrated beams onto boiler tubes. Ylla-Conte uses a large sectional convexplanar lens or a large stepped lens to track the sun. Dandini uses multiple small convex lenses mounted on a large transparent sphere to project concentrated beams inward to a central spherical receiving element through which liquid sodium is pumped.